Can You Mix 5W30 and 10W40 Oil [You Must Know]

Can you mix 5W30 and 10W40 oil? You can surely mix. But the real question is should you do it? Whether mixing 5w30 and 10w40 oil is safe or not depends on a few factors such as the brand of the engine oil, the type of engine, and the weather conditions.

To find out more about the safety of mixing 5w30 and 10w40 oil in detail, keep on reading.

Can You Mix 5W30 and 10W40 Oil [You Must Know]

You need to understand the difference between 5w30 and 10w40 oil first before taking a look at the factors that influence the safety of mixing them.

Note: You can also read If you can mix 10w30 and 10w40 or not.

What Is The Difference Between 5w30 and 10w40 Oil?

Both 5w30 and 10w40 are multigrade oils. Multigrade oils refer to oils that can change their viscosity depending on the surrounding environment.

The Win the oil reading stands for winter (cold). The viscosity of 5w30 oil is 5 and that of 10w40 oil is 10 at room temperature. Both these oils are capable of changing their viscosity when the temperature rises.

5w30 oil changes its viscosity to 30 at 100 degrees celsius. Now, you can understand what 10w40 oil means. The 10w40 oil will change its viscosity from 10 to 40 when the environment becomes warmer.

To summarize, 5w30 oils mean that the oil acts as 5W in winter and SAE 30 in summer. For 10w40 oil, it acts as 10W oil in winter and SAE 40 in summer.

How Do Oils Change Their Viscosity?

All of the oils have a base weight which determines their viscosity. For 5w30 oil, its base viscosity is 5. To maximize performance, additive packages are added to the base oil. Additive packages don’t change the viscosity.

The viscosity of the oil is changed by adding viscosity index improvers (VIIs). These VIIs are long-chain polymers that have the ability to change the viscosity of the oil depending on the weather.

Factors To Consider Before Mixing Oils

You should consider the following factors before mixing any engine oil:

  • The brand of the oil.
  • The engine of the vehicle.
  • The recommendation of the manufacturer.
  • The weather conditions.

Why Does Mixing Oil Depend on Brand?

As you talked about earlier, the multigrade oils are converted from single weight oil to multigrade oil by adding the following things:

  • Additive Packages
  • Viscosity Index Influencers

Different brands use different chemicals as additive packages and VIIs. If you’re mixing multigrade oil of different brands, the additives and VIIs may not be compatible. 

The additives of the different brands can clash with each other and lower the performance of your engine. The life cycle of the oils can be also be reduced if you’re using oil from different brands.

Generally, you shouldn’t mix oils. But if you must do it, try to mix oils of the same brand as they tend to be similar in chemical composition. They are likely to blend well with each other.

Why Does Oil Mixing Depend on The Engine?

Whether it is safe or not to mix oils of different grades depends on the engine. In a 5w30 engine, you can add 10w40 oil without seeing any major negative impact.

On the other hand, if you add 5w30 oil to a 10w40 engine, you’re taking a bigger risk. 5w30 oil is lower in viscosity and there is a small chance of the car not running. But if you’re adding less than a quart, it’ll be alright.

But because 5w30 and 10w40 oils are very close to each other, the chances of any negative consequences are slim. If you are using a 40w oil on a 5w30 engine, that is when you’re running a real risk of engine failure.

Why Follow The Manufacturer’s Guide Before Mixing Oil?

If you want to know whether it is safe to mix oil in your engine, the best place to look at is your manufacturer’s guide.

The manufacturers of your car and engine test a thousand different things before recommending which oil is the best for it. For example, a truck with a “5.7 Hemi Liter” engine recommends that you use 5w20 oil.

Sometimes, the viscosity of the oil needs to change depending on the weather conditions. That’s why most manufacturers provide a range of different oils that you can use.

Some manufacturers will tell you to use 10w40 during summer and 5w30 during the winter. It is ideal to follow your manufacturer’s recommendations to enjoy the optimum performance of your vehicle.

Why Does Mixing Oil Depend on Weather Conditions?

Weather conditions in an environment dictate what the viscosity of the oil should be. The very purpose of making multigrade oils is to give oils the ability to adjust to weather changes.

It can be a good idea to mix 10w40 and 5w30 oils when you’re driving in a hotter environment (if your manufacturer recommends it). But it’s a bad idea to do so in a colder climate. If you put 10w40 oil in a 5w30 engine in a cold climate, your car may not start because of the higher viscosity of the 10w40 oil.

How Much Oil Mixing Is Safe?

If you are mixing less than a quart of engine oil, you’re safe. Even if your manufacturer allows you to mix oils, it is safer to stick with one type of oil in the long run.

You should only consider mixing oils if you’re in a state of emergency or have adverse weather conditions. Still, don’t make a practice of doing it and keep the mixing to a minimum.


Do you get blended viscosity when you mix 5w30 and 10w40 oil?

Getting blended viscosity depends on the brand of oil you’re using. If you’re using the same brand, the oils are likely to blend together as the same additives and VIIs are used. Otherwise, they won’t blend together.

Can you mix synthetic oil with regular oil?

You can mix synthetic oil with regular oil. But it will most likely lead to an inferior performance from the synthetic oil.


Can you mix 5w30 and 10w40 oil? – You can but it may not be the best thing to do. The two different oils may not be compatible with each other.

Follow the manufacturer’s instructions to find out the best practices for your car engine. Leave us a comment if reading this post could help you or not.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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